3.1 – Microstructural Characterization
Figure 10 shows the micrographs for the DSS along the tube’s length. It is possible to verify the sporadic presence of large austenite islands, as well as large ferrite islands. Therefore, it can be said that the phase distribution along the length of the straight tube in the as received condition is not totally homogeneous. No intermetallic phases were found.
In Fig. 11, it shows the solution treated pipe in three different temperatures for three holding times, totaling nine solubilizing combinations. It is possible to note the presence of large random austenite islands at temperatures of 950 and 1060°C. In solubilized samples at 1150 ° C, the presence of the darker phase, ferrite, is more evident, which indicates a bigger concentration of ferrite at higher temperatures A similar microstructures observation was verified by Jiang et al. , which submitted the same DSS in solution treatments between 1000ºC and 1200ºC during 24h. No intermetallic phases were detected with Beraha solution.
Figure 12 shows the micrographs for the induction bent pipes at three temperatures and three different speeds, totaling nine combinations of bending. The Micrographs of curved samples indicated in Fig. 12 shows to be very similar to each other, as noted by . It was shown that the austenitic islands found themselves slightly smaller in curved samples with speed of 0.8 mm/s at the three tested temperatures, corroborating the statements of  that high heating rates prevent the grain growth and induce the formation of an ultra-fine microstructure after subsequent rapid cooling. Relatively large islands and random austenite were observed in curved samples, as well as in solubilized samples. The STB samples did not present a higher ferrite content at 1150ºC, oppositely to ST ones.
The results for the chromium nitride analysis are presented in Fig. 13, for both the solution treated and induction bent specimens at 1150°C. It is possible to observe the presence of chromium nitrides (Cr2N), indicated by the red arrows in the three samples solubilized by induction heating. It is important to note that the higher the solution treatment time at 1150 ° C, the higher the concentration of chromium nitrides. This is explained by the lower concentrations of austenite, which favors the segregation of nitrogen in the ferrite phase and the precipitation of Cr2N when cooling. Similar results were also found by [10–12]. The bent tubes (STB) showed a lower amount of chromium nitrides precisely for this reason, as can be seen comparing Figs. 11 and 12, where there was a more prominent presence of the ferrite at 1150ºC for the ST samples.
Figure 14 shows the ST sample at 950°C for 24 minutes and attacked with KOH for sigma phase detection. As can be noted at the referred figure, there were no signs of sigma phase. This result was corroborated by [12, 13], where only treatments for periods longer than 10h started to show appreciable sigma phase amounts.
3.3 –Phase Quantification, Toughness and CPT measurements
Figure 17 shows the results for toughness, ferrite concentration and CPT of the solution treated samples at 950°C for 3, 6 and 24 minutes. The dotted line, labelled AR in Fig. 17, identifies the as received condition.
Figure 17 indicates that there were no significant variations in the ferrite concentrations and toughness in the 950°C solubilized samples at all three holding times, when compared to the as received condition. The most obvious variation occurred in the sample that was solution treated for 24 minutes, where it is possible to note a slight decrease in the ferrite concentration and, proportionally, a slight increase in toughness. Regarding the CPT, the solution treated samples presented higher corrosion resistance when compared to the as received condition. Many studies [13, 15] have demonstrated an inversion relation between the solution treatment time and CPT or impact energy for long periods. However, this has not been observed for durations of less than one hour.
In the case of Fig. 18, it shows the outcomes for toughness, ferrite concentration and CPT for the bent samples at 950°C in 0.2, 0.5 and 0.8 mm/s speeds. The dotted line, labeled as AR in Fig. 18, represents the pipe in the as received condition.
It is evident from the results of Fig. 18, except for the toughness in the bent sample at the speed of 0.5 mm/s, that the values are consistent with each other. The discrepancy in the stated measurement can be explained by the results presented in Fig. 10, where it can be clearly seen that the ferrite concentration varies in the as received condition. In the case of specimen bent in 0.5 mm/s speed, the percentage of ferrite could have varied greatly, causing the discrepant measurement when compared to the other samples. Nevertheless, there was no significant variation between the values of toughness from the other bent samples when compared to the sample as received. The same statement is true for the ferrite content. Finally, the curved samples showed slight increase in pitting corrosion resistance as it increased the bending speed. This result can be explained by the lower time spent at the temperature of 950°C, preventing the precipitation of sigma phase.
Figure 19 shows the results for toughness, ferrite concentration and CPT of the solution treated samples at 1060°C for 3, 6 and 24 minutes.
It is shown in Fig. 19 that the sample solution treated at 1060°C for 6 minutes presented a CPT superior to the other measured, when it was expected to fall due to the reduced absorbed energy at impact test. However, the maximum difference among the CPTs of solubilized samples to 1060°C was 10.9°C which, in practical terms, is not significant.
In the case of Fig. 20, it shows the results for toughness, ferrite concentration and CPT for the bent samples at 1060°C in 0.2, 0.5 and 0.8 mm/s speeds.
Through Fig. 20, it can be noted a small variation between the ferrite concentrations and toughness from the bent samples and the sample as received. However, it is possible to note a slight raise in pitting corrosion resistance with increasing bending speed. This is an advantage of induction bending as manufacture process, when compared with welding, as the latter usually reduces CPT, even when mechanical properties values were acceptable [2, 16]. Moreover, the analysis of Figs. 19 and 20 shows that induction bending at 1060ºC tends to enhance pitting corrosion resistance, toughness and decrease ferrite content when comparing to solution treatments at the same temperature.
Figure 21 shows the results for toughness, ferrite concentration and CPT of the solution treated samples at 1150°C for 3, 6 and 24 minutes.
Figure 21 shows that solubilized samples at 1150°C presented higher values than the sample as received, which is justifiable due to the higher temperature. Both the absorbed impact energy and the CPT showed lower values than the sample as received, due to the presence of chromium nitrides in the ferrite phase at these samples, as presented in Fig. 13. Similar behavior was also verified by [10, 11, 17]. Summing up, when the same treatment time was considered, an increase in temperature resulted in higher ferrite content with a consequent CPT and toughness reduction.
Finally, Fig. 22 shows the results for toughness, ferrite concentration and CPT for the bent samples at 1150°C in 0.2, 0.5 and 0.8 mm/s speeds.
Through Fig. 22 it can be shown that the bent samples at 1150°C showed CPT values higher than the sample as received, despite evidence of precipitated chromium nitride in the ferritic phase. When comparing the induction heating treatments presented at Fig. 21 with the bent specimens presented in Fig. 22 it comes clear that ST led to rise in ferrite content, which resulted in a reduced absorbed energy and CPT, while induction bending enhances these properties. Some works [12, 14] has displayed that induction bending process could be deleterious to mechanical properties and corrosion resistance, being necessary a solution treatment at 1120ºC for at least 15 min to restore them. However, this work shows that solution treatments at higher temperatures during long times can also result in deleterious effects due to ferrite content increase and consequently chromium nitrides precipitation. Furthermore, it was detected that induction bending improves CPT, toughness and austenite content when compared with induction heating treatment.